This invention relates to an optical switching apparatus applied wavelength division multiplexing techniques thereto.
The optical switching apparatus switches optical signals without converting them into electrical signals. Recently, such optical switching apparatus might be expected to realize switching with a large amount of capacity which can not be realized by an electrical switching apparatus. In the art of optical switch, there are the following types: an optical space switching (or an optical space-division switching), an optical wavelength switching (or an optical wavelength-division switching), an optical time switching (or an optical time-division switching), and combinations thereof, such as an optical wavelength/space switching (an optical wavelength-division/space-division switching) and an optical wavelength/time switching (an optical wavelength-division/time-division switching).
In the optical space switching, channels are assigned only to spaces. FIG. 1 shows an example of an optical space switching apparatus. In the example zeroth through third input channels are assigned to zeroth through third input ports 10-0 through 10-3, respectively. On the other hand, zeroth through third output channels are assigned to zeroth through third output ports 11-0 through 11-3, respectively. Such optical space switching apparatus 1 can connect between any input channel and any output channel.
The optical space switching apparatus which the present invention relates, comprises an existing optical space switch of splitter/combiner type where semiconductor laser amplifiers are used as optical gate switches. For example, such switch is disclosed in Yoshiharu Maeno et al xe2x80x9cThe Possibility of Optical Switching Technology for Parallel Processing Systemsxe2x80x9d, IEICE, SB-9-5, 1996.
FIG. 2 illustrates an optical switch of splitter/combiner type known to the inventors. The illustrated optical space switch comprises zeroth through third input waveguides 20-0 through 20-3, zeroth through third beam splitters 21-0 through 21-3, zeroth through fifteenth optical gate switches 22-0 through 22-15, zeroth through third beam combiners 23-0 through 23-3, and zeroth through third output waveguides 24-0 through 24-3.
One kind of the existing optical gate switches is a semiconductor laser amplifier, which is turned Into a light-transmitting or an on state and a light absorbing or an off state when an electric current is fed thereto and is not fed thereto, respectively. For example, when the zeroth optical gate switch 22-0 turned into the on state, the zeroth input waveguide 20-0 is connected to the zeroth output waveguide 24-0.
The splitter/combiner type optical switch is strictly nonblocking and serves as a so-called crossbar switch where every pairs of input and output ports have dedicated connection paths. And accordingly, the optical space switching apparatus comprising the above switch also serves as a crossbar network. On the other hand, the optical switch of splitter/combiner type requires optical gate switches, (the number of ports)2 in number, and therefore, has a fault that it is difficult to be implemented, as the number of ports becomes large.
FIG. 3 shows another optical space switching apparatus known to the inventors. The apparatus is applied a wavelength division multiplexing (WDM) technology thereto, and achieves to reduce the number of the optical gate switches as compared with the apparatus illustrated in FIG. 2. In this apparatus, zeroth through fifteenth input optical signal each having any one of zeroth through third wavelengths xcex0 through xcex3 are supplied from zeroth through fifteenth input ports 10-0 through 10-15 and combined by zeroth through third beam combiners 31-0 through 31-3.
In detail, when the zeroth through third input optical signals having zeroth through third wavelengths xcex0 through xcex3 are input to the zeroth beam combiner 31-0 from the zeroth through third input ports, the zeroth beam combiner 31-0 combines the zeroth through third input optical signals to produce a zeroth WDM optical signal. Likewise, the first beam combiner 31-1 combines the fourth through seventh input optical signals having zeroth through third wavelengths xcex0 through xcex3 to produce a first WDM optical signal. The second beam combiner 31-2 are input the eighth through eleventh input optical signals having zeroth thereto from third wavelengths xcex0 through xcex3 from the eighth through eleventh input ports, and then, combines the eighth through eleventh input optical signals to produce a second WDM optical signal. The third beam combiner 31-3 combines the twelfth through fifteenth input optical signals having zeroth through third wavelengths xcex0 through xcex3 to produce a third WDM optical signal.
The optical space switch 32 illustrated in FIG. 3 is of a 4xc3x9716 crossbar switch adapted to perform 1-to-4 multicasting at maximum. The illustrated switch 32 has zeroth through third input ports i0 through i3 to which the zeroth through third WDM optical signals are supplied and zeroth through fifteenth output ports o0 through o15 from which zeroth through fifteenth switched WDM optical signals are outputted. The zeroth through fifteenth output ports of the optical space switch 32 are connected to zeroth through fifteenth wavelength selectors 33A-0 through 33A-15, respectively. The zeroth through fifteenth wavelength selectors 33A-0 through 33A-15 select the optical signal of the desired wavelengths from the zeroth through fifteenth switched WDM optical signals outputted from the optical space switch 32 and produce zeroth through fifteenth selected optical signals. The zeroth through fifteenth wavelength selectors 33A-0 through 33A-15 are connected to zeroth through fifteenth output ports 11-0 through 11-15, respectively. The zeroth through fifteenth output ports 11-0 through 11-15 transmit the zeroth through fifteenth selected optical signals as zeroth through fifteenth output optical signals, respectively.
Thus, the optical space switching apparatus has a function of a 16xc3x9716 crossbar network. In the apparatus, the optical space switch 32 may be of splitter/combiner type described above, and may include sixty-four optical gate switches.
On the other hand, each of the existing wavelength selectors 33A (suffixes omitted) comprises optical gate switches, the number of which is equal to the number of wavelengths transmitted into each selector. In the example described above with FIG. 3, the number of wavelengths multiplexed into the switched WDM optical signal is equal to four, and therefore, the number of optical gate switches is also equal to four. Specifically, in each selector, a wavelength demultiplexer demultiplexes switched WDM optical signal into individual optical signals with different wavelengths and transmits the individual optical signals into the optical gate switches, respectively. And then, one of the gate switches corresponding to desired wavelength turns on while the others turn off so that only the optical signal with desired wavelength is outputted from the selector.
As understood from the above, the optical switching apparatus of space division type illustrated in FIG. 3 has 128 optical gate switches In total. On the other hand, another 16xc3x9716 apparatus consisting of a splitter/combiner type optical switch requires 256 optical gate switches. Thus, the number of optical gate switches which comprise the apparatus illustrated in FIG. 3 is reduced to 1/2 as compared with another apparatus consisting splitter/combiner type optical switch.
As against the above optical space switching, optical wavelength/space switching assigns channels to both of wavelengths and spaces. FIG. 4 shows another example of an optical wavelength/space switching apparatus. In the example, zeroth through third input channels are assigned to zeroth and first input ports 10-0 and 10-1 and zeroth and first wavelengths xcex0 and xcex1 of optical signals transmitting on each of input ports. On the other hand, zeroth through third output channels are assigned to zeroth and first output ports 11-0 and 11-1 and zeroth and first wavelengths xcex0 and xcex1 of optical signals transmitting on each of output ports. Such optical wavelength/space switching apparatus 2 can connect between any input channel and any output channel. As a related technique, an optical wavelength/space switching apparatus having small sized-hardware is disclosed in Japanese Unexamined Patent Publication No. Hei 3-219793, namely, JP-A 3-219793 and is incorporated herein by reference.
One type of such apparatuses known to the inventors is modified the optical space switching apparatus illustrated in FIG. 3. The optical wavelength/space switching apparatus known to the inventors has no beam combiners 31-0 through 31-3 as preliminarily processing of the optical space switch 32 and directly are input WDM optical signals to the input ports of the optical space switch through the input ports. Besides, the optical wavelength/space switching apparatus has, as latter stage of the wavelength selectors, wavelength converters corresponding to the wavelength selectors 33A-0 through 33A-15 and beam combiners.
Furthermore, related techniques are disclosed in Japanese Unexamined Patent Publications Nos. Hei 7-59127, Hei 1-109991, Hei 3-100526, and Hei 2-27892, namely, JP-A 7-59127, JP-A 1-109991, JP-A 3-100526, and JP-A 2-27892, respectively. These related techniques are incorporated herein by reference.
The present invention provides optical switching apparatuses improved in various aspects, such as the size of hardware and the performance thereof.
Research has been directed to relationship between the number of wavelength multiplexed into each WDM optical signal transmitted to the input port of optical space switch and the size of hardware, in particular, the number of optical gate switches. As a result of research, the inventors have found out one thing that the number of optical gate switches required by an optical switching apparatus is optimized if the number of WDM optical signals responding to the input ports of the optical space switch and the number of wavelengths of each WDM optical signal is equal to each other in particular optical switching apparatuses, such as illustrated in FIGS. 2 and 3. This is common to an optical switching apparatus of space division type and that of wavelength-division/space-division type.
However, the numbers of WDM optical signals and wavelengths depend on an environment to which the optical switching apparatus is adapted. And furthermore, it is getting things backwards to modify the environment which has already been defined into another environment which corresponds to the numbers of WDM optical signals and wavelength transmitted into the optical space switch. Thus, the numbers are restricted by the environment.
Therefore, the present invention provides the following method of delivering a plurality of WDM optical signals to a plurality of input ports of an optical space switch. The method comprises preliminarily processing the WDM optical signals in relation to the number of input ports of the optical space switch, by optically processing the WDN optical signals so that the number of processed WDM optical signals responding to the input ports and the processed number of wavelengths of each processed WDM optical signal is equal to each other.
Herein, the numbers of the Input ports, the processed number of wavelength, the WDM optical signals, the wavelengths multiplexed in each of the WDM optical signals are equal to K, J, N, and M, respectively, where K, J, N, and M are integers not less than two. Furthermore, the above method may comprise using the optical space switch which further has K*M*N optical gate switches and which is connected to M*N wavelength selectors each comprising M*N/K additional optical gate switches, after the preliminarily processing.
The above method may comprise preliminarily processing the WDM optical signals in relation to the number of input ports of the optical space switch, by optically processing the WDM optical signals so that M*N is equal to J*K.
According to one aspect of the present invention, in case where M is larger than N, the preliminary process comprises, responsive to N WDM optical signals each of which has M multiplexed wavelengths., carrying out wavelength routing for the N WDM optical signals to produce K routed WDM optical signals as the K input WDM optical signals, each of which has J(=M*N/K) multiplexed wavelengths.
According to another aspect of the present invention, in case where M is smaller than N, the preliminary process comprises three processes: responsive to N WDM optical signals each of which has M multiplexed wavelengths, grouping N WDM optical signals into K sub-groups which comprises N/K WDM optical signals; carrying out optical wavelength shifting for (Nxe2x88x92K) ones of N WDM optical signals to make wavelengths of the (Nxe2x88x92K) WDM optical signals different from each other at each K sub-group; and then carrying out wavelength division multiplexing for the (Nxe2x88x92K) WDM optical signals subjected to the shifting and K WDM optical signals not subjected to the shifting at each K sub-group to produce, as the K input WDM optical signals, K additional WDM optical signals each of which has J(=M*N/K) multiplexed wavelengths.
The present invention further provides an optical switching apparatus which comprises an optical converter, an optical space switch, and wavelength selectors, as the followings. Responsive to N WDM optical signals each of which has M multiplexed wavelengths, the optical converter converts the N WDM optical signals into K input WDM optical signals each of which has J multiplexed wavelengths, wherein all of N, M, K and J are integers not less than two and J*K is equal to M*N. The optical space switch comprises K input ports and M*N output ports. The optical space switch responds to K input WDM optical signals to produce M*N switched WDM optical signals through the M*N output ports. The wavelength selectors is M*N. Responsive to the M*N switched WDM optical signals, respectively, the wavelength selectors selects one of J wavelengths multiplexed into the responding switched WDM optical signal.